DESCRIPTION

The gethostbyname*() and gethostbyaddr*() functions are obsolete.
Applications should use getaddrinfo(3) and getnameinfo(3) instead.
The gethostbyname() function returns a structure of type hostent for
the given host name. Here name is either a hostname, or an IPv4
address in standard dot notation (as for inet_addr(3)), or an IPv6
address in colon (and possibly dot) notation. (See RFC 1884 for the
description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no
lookup is performed and gethostbyname() simply copies name into the
h_name field and its structin_addr equivalent into the h_addr_list[0]
field of the returned hostent structure. If name doesn’t end in a dot
and the environment variable HOSTALIASES is set, the alias file pointed
to by HOSTALIASES will first be searched for name (see hostname(7) for
the file format). The current domain and its parents are searched
unless name ends in a dot.
The gethostbyaddr() function returns a structure of type hostent for
the given host address addr of length len and address type type. Valid
address types are AF_INET and AF_INET6. The host address argument is a
pointer to a struct of a type depending on the address type, for
example a structin_addr* (probably obtained via a call to
inet_addr(3)) for address type AF_INET.
The sethostent() function specifies, if stayopen is true (1), that a
connected TCP socket should be used for the name server queries and
that the connection should remain open during successive queries.
Otherwise, name server queries will use UDP datagrams.
The endhostent() function ends the use of a TCP connection for name
server queries.
The (obsolete) herror() function prints the error message associated
with the current value of h_errno on stderr.
The (obsolete) hstrerror() function takes an error number (typically
h_errno) and returns the corresponding message string.
The domain name queries carried out by gethostbyname() and
gethostbyaddr() use a combination of any or all of the name server
named(8), a broken out line from /etc/hosts, and the Network
Information Service (NIS or YP), depending upon the contents of the
order line in /etc/host.conf. The default action is to query named(8),
followed by /etc/hosts.
The hostent structure is defined in <netdb.h> as follows:
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
The members of the hostent structure are:
h_name The official name of the host.
h_aliases
An array of alternative names for the host, terminated by a NULL
pointer.
h_addrtype
The type of address; always AF_INET or AF_INET6 at present.
h_length
The length of the address in bytes.
h_addr_list
An array of pointers to network addresses for the host (in
network byte order), terminated by a NULL pointer.
h_addr The first address in h_addr_list for backward compatibility.

RETURNVALUE

The gethostbyname() and gethostbyaddr() functions return the hostent
structure or a NULL pointer if an error occurs. On error, the h_errno
variable holds an error number. When non-NULL, the return value may
point at static data, see the notes below.

ERRORS

The variable h_errno can have the following values:
HOST_NOT_FOUND
The specified host is unknown.
NO_ADDRESS or NO_DATA
The requested name is valid but does not have an IP address.
NO_RECOVERY
A non-recoverable name server error occurred.
TRY_AGAIN
A temporary error occurred on an authoritative name server. Try
again later.

FILES

CONFORMINGTO

POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(),
endhostent(), gethostent(), and h_errno; gethostbyname(),
gethostbyaddr(), and h_errno are marked obsolescent in that standard.
POSIX.1-2008 removes the specifications of gethostbyname(),
gethostbyaddr(), and h_errno, recommending the use of getaddrinfo(3)
and getnameinfo(3) instead.

NOTES

The functions gethostbyname() and gethostbyaddr() may return pointers
to static data, which may be overwritten by later calls. Copying the
structhostent does not suffice, since it contains pointers; a deep
copy is required.
In the original BSD implementation the len argument of gethostbyname()
was an int. The SUSv2 standard is buggy and declares the len argument
of gethostbyaddr() to be of type size_t. (That is wrong, because it
has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t,
which is OK.) See also accept(2).
The BSD prototype for gethostbyaddr() uses constchar* for the first
argument.
SystemV/POSIXExtension
POSIX requires the gethostent() call, that should return the next entry
in the host data base. When using DNS/BIND this does not make much
sense, but it may be reasonable if the host data base is a file that
can be read line by line. On many systems a routine of this name reads
from the file /etc/hosts. It may be available only when the library
was built without DNS support. The glibc version will ignore ipv6
entries. This function is not reentrant, and glibc adds a reentrant
version gethostent_r().
GNUExtensions
Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
permits to specify the address family to which the address must belong.
Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(),
gethostbyname_r() and gethostbyname2_r(). The caller supplies a
hostent structure ret which will be filled in on success, and a
temporary work buffer buf of size buflen. After the call, result will
point to the result on success. In case of an error or if no entry is
found result will be NULL. The functions return 0 on success and a
non-zero error number on failure. In addition to the errors returned
by the non-reentrant versions of these functions, if buf is too small,
the functions will return ERANGE, and the call should be retried with a
larger buffer. The global variable h_errno is not modified, but the
address of a variable in which to store error numbers is passed in
h_errnop.

BUGS

gethostbyname() does not recognize components of a dotted IPv4 address
string that are expressed in hexadecimal.